Absolute extinction curves of the spectroscopic GRB sample. The extinction curves are based on the best fit models given in Table 2. The grey curve represents the optical spectrum. The black triangles correspond to acquisition camera photometry used for scaling the afterglow SED. Black open circles are not included in the spectral fitting because of the optical spectrum wavelength coverage in that region while solid circles represents the photometric data points included in the SED modeling. The red solid curve corresponds to the best dust model for each GRB. Also shown are the Milky Way (green dashed line) and SMC (blue dotted line) extinction models taken from Pei (1992).